1. Field of the Invention
This invention relates to an image forming device such as a copying machine, a printer or a facsimile and, more particularly, to an image forming device using a developer with two components.
2. Discussion of the Background
A related image forming device such as a copying machine, a printer or a facsimile, includes a developer stirring and conveying member for stirring and conveying a two-component developer to a developing sleeve developing an electrostatic latent image formed on a photosensitive body; a new toner replenishing member for replenishing new toner to a toner conveying member provided through a toner conveying path that communicates with a developer stirring and conveying path of the developer stirring and conveying member; and a recovered toner conveying member for conveying recovered toner, removed from the photosensitive body, to the toner conveying member for use as recycle toner.
The developer stirring and conveying member includes a first developer stirring and conveying member positioned along and substantially in parallel to the developing sleeve and a second developer stirring and conveying member positioned substantially in parallel to the first developer stirring and conveying member. These members are designed to convey the developer in directions which are different from each other.
A partition is provided between the first developer stirring and conveying member and the second developer stirring and conveying member. Developer receiving portions are formed at front and back sides of the partition, respectively, so that the conveyed developer can be circulated through the developer receiving portions. The developer is conveyed in this circulatory system while substantially maintaining a constant speed even in the developer receiving portions on purpose to prevent the developer from accumulating or decreasing at the ends of each developer stirring and conveying member. If the first and second developer stirring and conveying members convey the developer at different speeds, e.g., when the conveying speed of the first developer stirring and conveying member is slower than that of the second developer stirring and conveying member, there exists no developer (especially no carrier) in the leading portion of the second developer stirring and conveying member; and only the replenished toner and the recycle toner are conveyed to the second developer stirring and conveying member. This may cause a toner concentration detecting sensor to make an error in detection.
The first developer stirring and conveying member and the second developer stirring and conveying member are each constructed by an elliptic fin member having a plurality of fins each formed by notching part of an elliptic plate and slantingly arranged, mainly aiming at stirring of the developer. This type of image forming device is required to stir the developer enough to charge the toner to a predetermined potential before development on purpose to prevent the occurrence of an uneven density of an image caused by insufficient mixing of the toner with the carrier of the developer; and further, to prevent the occurrence of smudges on the background of the image due to insufficient charge of the toner.
That is, the first developer stirring and conveying member and the second developer stirring and conveying member are required to convey the developer at a relatively low speed so that the developer can be stirred enough to charge the toner. For this reason, these members are each constructed by an elliptic fin member having a plurality of fins each formed by notching part of an elliptic plate and slantingly arranged.
The toner conveying path communicates with the second developer stirring and conveying member, and the toner conveying member provided through the toner conveying path is formed into a screw-like shape.
Re-use of the toner recovered from the photosensitive body as recycle toner allows a reduction in the number of times new toner is replenished. This is particularly effective in a compact image forming device employing such a device called a process cartridge or a photosensitive unit, in which image forming units such as a developing unit and a cleaning station arranged around the photosensitive body are unitarily formed with the photosensitive body, and which therefore is incapable of making the storage capacity of the developer and replenishing toner large.
The recycle toner is, however, used and recovered toner that has been removed from the surface of the photosensitive body, and is fairly condensed compared to the new toner because the recovered toner is circulated and conveyed in the condition that it is pushed into the toner conveying means by the recovered toner circulating and conveying means. It is therefore difficult to mix such condensed recycle toner with the carrier of the two-component developer, and further, it is difficult to charge the recycle toner. For this reason, the reuse of the condensed recovered-toner as the recycle toner causes insufficient dispersion of the toner into the developer and hence uneven or unstable charge of the toner. Therefore a problem arises that smudges on the background of an image occur. Such tendency is remarkable in a compact image forming device employing the process cartridge or the photosensitive body unit, because it is incapable of making the developer stirring and conveying path long due to its construction.
Further, since the developer conveying speed is relatively slow in this type of image forming device, it is easy to accumulate the developer near the developer receiving portions.
Furthermore, in conventional image forming devices, the toner replenished from a toner replenisher is fed to the second developer stirring and conveying member through the toner conveying member provided separately from the second developer stirring and conveying member.
Since the developer near one of the developer receiving portions that communicates with the downstream of the developer circulating path of the first developer stirring and conveying member is relatively heavy with a large amount of carrier due to toner consumption, the developer is accumulated near the developer receiving portion in the conventional image forming devices. This may cause a backflow of the developer into the toner replenishing path or a slowdown in dispersing the toner into the developer after being fed to the second developer stirring and conveying member. In these cases, the toner concentration in a toner concentration detecting position reaches a reference value very slowly even after a required amount of toner has been replenished. This causes excess toner replenishment and therefore the same problem arises that smudges on the background of an image occur. The problem is remarkable in a compact image forming device.
Referring now to FIG. 27, a reason for the nonuniform mixing of the circulated developer with the replenished toner will be described from another standpoint. In the drawing, the developer is represented by circles with reference symbol G and the toner is represented by scattered dots with reference symbol Tn for convenience sake. A first developer stirring and conveying member 101 conveys the developer G to a second developer stirring and conveying member 104 (only a toner conveying member 102 is shown in the drawing) provided at the right side of the first developer stirring and conveying member 101 when viewed from the drawing. In this case, the first developer stirring and conveying member 101 rotates in a counterclockwise direction. The second developer stirring and conveying member 104 also rotates in a counterclockwise direction to stir and convey the toner Tn and the developer G toward the back side of the plane of the drawing.
The toner Tn replenished to the toner conveying member 102 is accumulated by gravity on a casing 103 and at the lower side of the toner conveying member 102. As shown in FIG. 27, the accumulated toner Tn is slid on the upper surface of the casing 103 and sent to the developer receiving portion as a screw vane 102a rotates in the lower portion of the toner conveying member 102. On the other hand, the developer G sent from the first stirring and conveying member 101 to the second stirring and conveying member 104 is fed along the casing 103 with a flat plane to the lower portion of the toner conveying member 102, as shown by an arrow 106 in FIG. 27, where the toner Tn is accumulated. The developer G is thus delivered from the first stirring and conveying member 101 to the second stirring and conveying member 104 through the developer receiving portion in FIG. 27 so that the developer G will join the toner Tn in the developer receiving portion.
When the developer G joins the toner Tn, the first stirring and conveying member 101 rotates to feed the developer G, having a weight which is heavier than the toner Tn, along the plane of the casing 103 into the lower portion of the second stirring and conveying member 104 as shown by the arrow 106. Since the toner Tn which is lighter in weight than the developer G is pushed up by the developer G, it is difficult to disperse the toner Tn into the developer G, and hence it is difficult to cause friction between the toner Tn and the carrier included in the conveyed developer G so as to charge the toner Tn. The use of the toner insufficiently charged for development causes defective development such as smudges on the background of an image. If toner is replenished with the toner concentration detected from the toner that has not yet been charged enough with friction, proper toner concentration cannot be obtained.
In such a related image forming device, the replenished toner is fed from the upper side of the end of the second developer stirring and conveying member through a toner replenishing port. The replenished toner falls on the screw vane. Since the developer stirring and conveying member generally rotates at a speed of 200 rpm or higher and the toner consists of micro-particles of 3 to 12 .mu.m, some particles of the replenished toner cannot fall from the toner replenishing port to the lower side, and may be scattered and suspended in the air or moved on the surface of the developer without being mixed and stirred with the developer, particularly with the carrier. Such toner directly arrives at the developing sleeve and the developer around the developing sleeve without being charged or with weak charging and causes smudges on the background of an image. Such scattering of the toner also causes stains in the device.
Further, in an image forming device with such a construction, the developer stirring and conveying direction is typically limited to the axial direction of the developer stirring and conveying member, and the location of the toner concentration detecting means (see Japanese patent published application No. 63-28305 or Japanese patent laid-open application No. 58-55952) for detecting toner concentration is also restricted. Therefore, in such an image forming device, the toner concentration in the developer differs depending upon the positions within the developing unit and the detecting result of detecting toner concentration depends on the position where the toner concentration detecting means is positioned within the developing unit. Also, the bulk of the developer is changed within the developing unit depending on the toner replenishment timing and the amount of replenished toner. The deviation of the toner concentration in the developer within the developing unit causes uneven density of the image, and it further causes smudges on the background of the developed image depending upon the toner replenishment timing and the amount of replenished toner. It may further cause an overflow of the toner excessively replenished depending on the toner replenishment timing and the amount of replenished toner.
An easy and typical way to solve the above problems is to extend the length of the developing casing or the partition. Such an approach to the conventional problems allows a prevention of the movement of the scattered or floating toner particles, but on the other hand could resist the movement of the developer. The approach is also insufficient for responding to a change in level of the developer surface caused when toner has been replenished.